allow you to set the underlying read timeout when initializing client

The default read timeout for ruby Net::HTTP is 60s If you are using
the watch feature in etcd you might want to set that much longer. This
patch allows you to set read_timeout: attrib at initialization of your
client, but keeps the default behavior of 60s
Merge pull request #2 from spheromak/read_timeout

allow you to set the underlying read timeout when initializing client